In recent years, a liquid crystal display has been widely used in various display devices owing to characteristics thereof, such as low power consumption, low voltage operation, lightweightedness, and slimness. A liquid crystal display may include many elements, such as a liquid crystal cell, a polarizing plate, a phase difference film, a prism sheet, a diffusion film, a light guide plate, a light reflection sheet, etc. To this end, by decreasing the number of films used or making the films or sheets thin, improvements targeted at enhancements in productivity, lightweightedness and brightness are actively being undertaken.
The polarizing plate includes a polarizer made of a polyvinyl alcohol (PVA) resin dyed with dichroic dye or iodine, and a protective film stacked on one surface or both surfaces of the polarizer with an adhesive provided therebetween. Until recently, a triacetyl cellulose (TAC) film has been widely used as the protective film. However, the TAC film has a drawback in that it may be deformed in a high temperature and high humidity environment. Therefore, in recent years, protective films made of various materials capable of replacing the TAC film have been developed, and, for example, the use of polyethylene terephthalate (PET), a cycloolefin polymer (COP), an acrylic film alone or mixtures thereof, has been proposed.
Meanwhile, examples of adhesives that may be used for attaching the polarizer and the protective film to each other include an acrylic adhesive, a dry laminated adhesive made by mixing a urethane resin solution and a polyisocyanate resin solution, a styrene/butadiene/rubber adhesive, an epoxy adhesive, a polyvinyl alcohol adhesive, a urethane adhesive, an adhesive containing a compound having a polyester ionomer-type urethane resin and a glycidyl group, a thermosetting adhesive, or the like, while water-based adhesives made of an aqueous solution of polyvinyl alcohol resin may chiefly be used.
However, in the case of the water-based adhesives, when an acrylic film or a COP film is used as the protective film instead of the TAC film, the adhesive force of the protective film is weak, and thus, the protective film has a drawback in that use thereof is limited according to the material of the protective film. Also, in the case of water-based adhesives, in addition to adhesive failure due to the material, when the protective films applied to both surfaces of a PVA base film are made of different materials, curling of the polarizing plate may occur in the course of a drying process of the water-based adhesives and lowering of initial optical properties may be caused. As an alternative to solve these drawbacks, a non-water-based adhesive has been proposed.
However, since non-water-based adhesives generally have a high viscosity, a final adhesive layer shows a relatively thick tendency, compared with that in water-based adhesives. If the thickness of the adhesive layer is increased, a failure, such as curling occurs in a TD direction and an MD direction. Thus, it is preferable to reduce the thickness of the adhesive layer.
Therefore, in order to minimize the viscosity of the adhesive layer, a method of adding a hydrophilic monomer to a non-water-based adhesive composition has been proposed. However, the addition of the hydrophilic monomer may decrease water resistance. The decrease in water resistance may be solved by adding multi-functional monomers or hydrophobic monomers, but the addition of multifunctional monomers or hydrophobic monomers may lead to a decrease in adhesive force. As a solution to solve a reduction in water resistance due to the addition of hydrophilic monomers, a method of adding an isocyanate compound has been proposed, but such isocyanate compounds may cause problems in terms of solution stability, due to a high reactivity with hydrophilic monomers.
As another method to lower the viscosity of the non-water-based adhesive, a method of decreasing the amount of oligomers and adding single functional monomers may be considered, but this method lowers the glass transition temperature of adhesives to weaken the heat resistance and water resistance of the adhesives. Meanwhile, if the glass transition temperature of the adhesive is lower than that of a polarizer, a failure, such as cracking of the polarizer may occur. Although a method of adding multifunctional monomers or monomers having a high glass transition temperature to a homopolymer may be considered to improve the reductions in heat resistance and water resistance of adhesives, the method may decrease the adhesive force.
Therefore, there is a need to develop a non-water-based adhesive for a polarizing plate which has superior adhesive force with respect to films of various materials, for example, a TAC film, an acrylic film, a COP film, a PET film, and the like, and a low viscosity, making thin adhesive layers, and has superior water resistance and heat resistance (thermal impact resistance).